Acanthostega: The shape of things to come

File:Acanthostega MLCS.JPG - Wikipedia
Acanthostega wondering what you’re looking at!
Image Credit: Conty, https://en.wikipedia.org/wiki/File:Acanthostega_MLCS.JPG

Throughout earth’s history there have been many major leaps in evolution; the evolution of eyes, the first multicellular animals and (from our point of view) the first time our hominid ancestors walked upright. However one that sticks out in a few people’s minds is when vertebrates first hauled themselves out of the water and started walking on land. To illustrate just how big this step was, imagine an alternate reality where it never happened. This parallel world would contain no reptiles, birds or mammals, and human civilisation would’ve never emerged. The vertebrates in this world are comprised of a wide variety of fish species swimming in seas, lakes and rivers across the world alongside a range of molluscs, crustaceans and corals (to name a few). On land the world is still covered in thousands of plant species but the only animals are invertebrates. Beetles, arachnids, and ants of all possible sizes scuttle along the ground. Dragonflies, wasps and flies buzz and dance through the air and worms bury through the soil keeping the ecosystem together. All in all, it is a world radically different to what we know.

As a result documenting how and why this important moment in life on earth occurred is key to understanding the world around us. One animal that has helped palaeontologists to do this is a 60 cm long stem-tetrapod that swam the rivers of Greenland during the Devonian Period (360 million years ago). Its name was Acanthostega gunnari, meaning “Gunnars spiny roof”.

File:Acanthostega model.jpg
A model of a swimming Acanthostega
Image Credit: Dr. Günter Bechly, https://commons.wikimedia.org/wiki/File:Acanthostega_model.jpg

While fossils of Acanthostega were first discovered in 1933 (and described in 1952 by Gunnar Säve-Söderbergh, who the species is named after, and Erik Jarvik) the majority of what we know about it comes from a magnificent bone bed, part of the Celsius Bjerg Group rock sequence found in East Greenland, that was discovered in 1987 by a team led by Palaeontologist Jenny Clack. These beds contains the remains of multiple Acanthostega buried and preserved with their skeletons almost completely intact. A paper released in 2016 (Sanchez et. al. 2016) detailed another interesting observation about these fossils. Micro CT synchrotron scans of the interior of the arm bones showed that the bones were still reasonably cartilaginous and had yet to fully ossify (i.e. harden into fully formed bones). This ossification happens as animals mature, so it was deduced that all of the 1987 fossils were of juvenile Acanthostega (roughly around 6 years old) who seem to have been living together. The ossification process also seemed to have progressed further in some individuals than in others, suggesting that there was size variation between members of the group, either through genetic variation, sexual dimorphism or even both. Tragically for this ragtag group of youngsters, it seems that they all died together. It is thought that a flash flood might have washed all of them into a small pool of water. This then dried up after the flood receded leaving them stranded and exposed to the elements, away from the water that kept their skin from drying out.

Acanthostega is a great example of a transitional fossil. Its anatomy is comprised of both basal fish-like features (e.g. internal gills, fish-like teeth, fleshy tail fins and a lateral line system) and derived tetrapod-like features (e.g. simple lungs and limbs tipped with digits). Curiously all these features would have made Acanthostega perfectly suited for its river home. It used its fleshy tail to power itself through its river home, snapping at any fish that wondered too close, and to help locate its prey and navigate through its watery environment it used a lateral line system to sense movement and pressure gradient changes. These features (along with its internal gills) meant it stayed underwater for long stretches of time, though its simple lungs enabled it to take breathes of air if required. What surprised palaeontologists the most about Acanthostega was the structure and function of its limbs and the number of digits on each limb. The limbs were not large or robust enough to bear Acanthostega’s weight for long, meaning it would only rarely spend time on land (if at all). Instead the limbs acted as paddles, aiding with swimming and manoeuvring underwater. This is important because it showed that the early tetrapods didn’t evolve limbs when they started walking on land, but instead first evolved them to better aid them underwater. Then later in time they would adapt this pre-existing feature to use for walking on land. The story is the same for its digits. Each of Acanthostega’s limbs were tipped with 8 digits. This showed that the number of digits on stem-tetrapod limbs wasn’t restricted to a set number (originally thought to have been 5). These early digits would have had webbed and made the early limb a more effective paddle. Then later in evolutionary time digits (like limbs) evolved to help bear and spread out the vertebrate’s weight when it was on land.

File:Acanthostega gunnari.jpg - Wikimedia Commons
A skeletal of reconstruction of Acanthostega. Note its 8 digits, flat skull and paddle-tail.
Image Credit: Ryan Somma, https://commons.wikimedia.org/wiki/File:Acanthostega_gunnari.jpg

Looking at all of its features its certain that Acanthostega would have actually spent almost all of its time in water, patrolling the waterways and hunting for small fish and arthropods. Its fish like skull features and relatively weak bite force (adapted more for gripping prey) compared to later tetrapods were perfectly adapted for catching slippery aquatic prey, meaning it didn’t hunt terrestrial animals. Like modern day amphibians Acanthostega would have laid its eggs in water as the eggs lacked a hard watertight casing. Throughout its life Acanthostega would also have had to watch its back! Multiple species of large freshwater fish were alive during the Late Devonian and many of them would have seen Acanthostega as a tasty meal.

So while Acanthostega wouldn’t have been much of a “land lubber”, it was a shape of things to come. This small river dweller helped palaeontologists to figure out the early evolutionary history of the stem-tetrapods and showed that limbs and digits, those features that you use every day, were first developed for underwater use, and only later on evolved for use on land.

All that we know about Acanthostega, the evolution of limbs and digits and how vertebrates first ventured out of the water, couldn’t have been possible without the hard work and dedication of Jenny Clack. Before her work this evolutionary transition period wasn’t particularly well understood. However her meticulous research on every facet of Acanthostega (whose fossils she sometimes gave nicknames to, such as “Boris”, “Rosie” and “Grace”) and its relatives, revolutionised our understanding of this key period of vertebrate evolution. She was one of the world’s leading experts on stem-tetrapods and Acanthostega in particular. This is clear to see as almost every scientific paper released about Acanthostega over the last three decades has carried her name either as a researcher or as a source. Sadly Jenny Clack passed away in March of this year (at time of writing). She will be greatly missed by her friends, family and the wider scientific community. With her passing, the world has lost one of the great palaeontologists.

References/Further Reading

Sanchez et. al. 2016 paper detailing the growth and life history of Acanthostega

Sanchez, S., Tafforeau, P., Clack, J. et al. Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography. Nature 537, 408–411 (2016). https://doi.org/10.1038/nature19354

Clack 2002 paper on the skull roof of Acanthostega

Clack, J. (2002). The dermal skull roof of Acanthostega gunnari, an early tetrapod from the Late Devonian. Transactions of the Royal Society of Edinburgh: Earth Sciences, 93(1), 17-33. doi:10.1017/S0263593300000304

Neenan et. al. 2014 paper on the feeding biomechanics of Acanthostega

James M. Neenan, Marcello Ruta, Jennifer A. Clack and Emily J. Rayfield (2014) Feeding biomechanics in Acanthostega and across the fish–tetrapod transition, Proc. R. Soc. B.28120132689, https://doi.org/10.1098/rspb.2013.2689

Porro, Rayfield & Clack 2015 paper on a 3d reconstruction of an Acanthostega skull. This allowed the trio to infer how Acanthostega caught prey.

Porro, Laura B et al. (2015) “Descriptive anatomy and three-dimensional reconstruction of the skull of the early tetrapod Acanthostega gunnari Jarvik, 1952.” PloS one vol. 10,3 e0118882, doi:10.1371/journal.pone.0118882

Tree of Life web project section on Acanthostega, written by the Late Jenny Clack

Clack, Jennifer A. 2006. Acanthostega. Acanthostega gunnari. Version 13 June 2006. http://tolweb.org/Acanthostega_gunnari/15016/2006.06.13 in The Tree of Life Web Project, http://tolweb.org/

Another Tree of Life project section written by Jenny Clack on the definition of “Tetrapod” and how it is debated

Clack, Jennifer A. 1997. The Definition of the Taxon Tetrapoda, 1997, http://tolweb.org/accessory/Definition_of_the_Taxon_Tetrapoda?acc_id=471 in The Tree of Life Web Project, http://tolweb.org/

The University of Cambridge Department of Zoology news article on the passing of Professor Jenny Clack

Aucott, Rachael, “Professor Jenny Clack, FRS, 1947-2020”, University of Cambridge, 26th March, 2020, https://www.zoo.cam.ac.uk/news/professor-jenny-clack-frs-1947-2020

A Science Direct web page about lateral line systems

Science Direct, “Lateral Line System”, Science Direct, https://www.sciencedirect.com/topics/medicine-and-dentistry/lateral-line-system

Clack & Neininger 2000 paper on the Celsius Bjerg Group, a rock sequence that Acanthostega fossils have been found in

Clack, J. A. and S. L. Neininger (2000). “Fossils from the Celsius Bjerg Group, Late Devonian sequence, East Greenland; significance and sedimentological distribution.” Geological Society, London, Special Publications 180(1): 557-566.

Pentecopterus: The oldest Sea Scorpion yet

File:Eurypterids Pentecopterus Horizontal.jpg
A pair of Pentecopterus exploring together
Image Credit: Patrick Lynch, https://commons.wikimedia.org/wiki/File:Eurypterids_Pentecopterus_Horizontal.jpg

In the heart of London a famous food critic, sporting a sharp suit and meticulously polished shoes, sits down at a table. He’s about to review a wildly popular new restaurant, where the signature dish is “The Sea-Food Surprise“. After a disappointing starter that he could only describe as “tasting of old, dried rock” the Surprise arrives. To the critics astonishment the dish is absolutely splendid! Cooked to perfection in a piquant sauce and with a texture and taste reminiscent of the best lobster ever! The next day he returns to the restaurant, demanding to be be shown the “lobster” he had had last night. “I’ll get the chef” the waiter replies. Eventually the chef emerges from the kitchen and asks the food critic to follow him. The critic passes a tank with a pair of North Atlantic Lobsters silently observing him with their beady stalked eyes. However the chef continues on past them. “If they aren’t the lobster then what is?” The critic wonders to himself. He is taken through the kitchen and down a set of stairs. “Surely you don’t keep lobster in the wine cellar?!” the critic joked. “You’re right, we don’t” the chef replies, opening a hatch in the wine cellar that leads to second, hidden cellar. In this cellar is a giant pool, and in this pool is a group of the “lobster” he had eaten the night They are huge, as long as a man and possessed a flat tail, smooth semi-circular head, spines sticking out all over the place and large claws which they were using to tear into the fish that were being used to feed them. It truly was unlike any seafood he had ever seen. “What in the world is that?!” The critic exclaimed! “The Sea-Food Surprise” The chef replied. “Pentecopterus, a Sea-Scorpion”.

An illustration of a Pentecopterus swimming between vegetation.
Image Credit: Apokryltaros, https://commons.m.wikimedia.org/wiki/File:Pentecopterus_decorahensis.JPG

While a story like this could only be from the realm of science fiction, Pentecopterus decorahensis (Greek for “Decorah’s Penteconter Wing”) was a very real animal. It belonged to an order known as the Eurypterids, although they’re more commonly known as “Sea-Scorpions”. While it is another example of a cool paleontological nickname, “Sea Scorpion” is misleading. While most (but not all) members of this order did live in the sea they were not scorpions, but instead were part of a completely separate group. That being said modern scorpions and other arachnids such as spiders, along with horseshoe crabs, are the Sea Scorpions closest living relatives. This is unsurprising as Sea Scorpions looked like what would happen if a scorpion and a crab decided that they wanted to settle down and start a family! Sea Scorpions were a highly successful order of animals during the Palaeozoic era, having a timespan from 467 Million Years ago (during the Mid-Ordovician period) to 252 Million Years ago, succumbing to extinction during the “great dying” at the end of the Permian period. This means that Sea Scorpions were alive for longer than the dinosaurs!

What makes Pentecopterus special among this already interesting group is that it is the oldest species of Sea Scorpion discovered so far. Their fossils are estimated to be about 467 Million Years Old. These fossils were first formally described in 2015 by a team led by James C Lamsdell, who named the animal after the penteconter, an Ancient Greek galley ship that, like the animal, possessed a sleek shape. The original fossils were discovered in 2010 in the Winneshiek Lagerstätte near Decorah in Iowa, USA (Hence its full name). Winneshiek is an interesting place as it is located within the remnants of an old meteorite crater! When Pentecopterus was alive this 5 km wide meteorite crater was filled with sea water, creating a unique shallow marine environment. The relatively still waters and lack of oxygen on the crater floor resulted in the perfect conditions for fossilisation, and the relatively low salinity of the water meant that marine animals typical of the Ordovician oceans couldn’t live here, creating a unique ecosystem for its time. As a result of these conditions multiple Pentecopterus individuals (ranging from juveniles up to fully grown adults) were perfectly preserved, so perfectly in fact that the outline of the tiny sensory hairs can be seen.

As you can see in this comparison, a Pentecopterus was one large arthropod!
Image Credit: Slate Weasel, https://commons.m.wikimedia.org/wiki/File:Pentecopterus_Size.svg

So what exactly did Pentecopterus look like and how would it have behaved? Pentecopterus, like all arthropods (a group of invertebrates containing all insects and arachnids), would have had an exoskeleton divided into a series of distinct segments. At the front was a round, smooth and sleek head with two pairs of eyes; (one pair of compound-eyes at the front and a pair of smaller eyes at the top). Its body had 3 pairs of legs attached to it, that it used for walking on the sea floor (and maybe occasionally on land), and behind them a pair of broad paddles (that were uniquely shaped to this Sea Scorpion) used to help it swim and turn in the water. Finally at the back end there was a long, broad tail (known as a telson) that it used to power itself through the water. Pentecopterus also possessed a pair of vicious claws at the front of its body that it used to grab prey, (consisting of early jawless fish species and other arthropods). Pentecopterus belonged to a larger family of Sea Scorpions known as the Megalograptids. These are characterised by possessing many more spines on their legs compared to other Sea Scorpions, giving Pentecopterus a very spiny look. This would have been an unnervingly large animal to look at, growing as long as a human is tall (1.8 metres). However there were later Sea Scorpions that could grow even larger than this, with the largest being the 2.5 metre long Jaekelopterus!

Such a magnificent animal would have certainly been very eye catching and, if modern day crabs and lobsters are anything to go by, would potentially have made for a unique sea food delicacy, as the food critic in our story found out!

References/Extra Reading

Lamsdell et. al. 2015 paper describing Pentecopterus

Lamsdell, J.C., Briggs, D.E.G., Liu, H.P. et al. The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa. BMC Evol Biol 15, 169 (2015). https://doi.org/10.1186/s12862-015-0443-9

Briggs et. al. 2018 paper reviewing the Winneshiek biota, home to Pentecopterus and other animals

Derek E.G. Briggs, Huaibao P. Liu, Robert M. McKay, Brian J. Witzke; The Winneshiek biota: exceptionally well-preserved fossils in a Middle Ordovician impact crater. Journal of the Geological Society ; 175 (6): 865–874. doi: https://doi.org/10.1144/jgs2018-101

A blog written by Palaeontologist Dave Marshall, and published in BMC Series blog, about the discovery of Pentecopterus and its significance

Marshall, Dave, “The oldest and the scariest sea scorpion: a giant discovery”, BMC Series blog, Sep. 1, 2015, blogs.biomedcentral.com/bmcseriesblog/2015/09/01/oldest-scariest-sea-scorpion-giant-discovery/

Another blog, written by Jim Shelton and published in YaleNews, on the discovery of Pentecopterus

Shelton, Jim, “Meet the newest ‘sea scorpion’: Pentecopterus, a predator from prehistoric seas”, YaleNews, Aug. 31, 2015, news.yale.edu/2015/08/31/meet-pentecopterus-new-predator-prehistoric-seas

Arthropleura: The Titan of the Carboniferous

Artists reconstruction of the Carboniferous Titan Arthropleura
Image Credit: Nobu Tamara, http://spinops.blogspot.com/

One day, two executives at Warner Brothers film studio are sitting in their office with their heads in their hands. The last film has massively under-performed at the box office, the writers are struggling for ideas, A-List actors are turning their noses up at working for them and their film sets are so quiet you can hear the crickets chirping. However there is a knock at the door. “Come in!” shouts one of the executives. A nerdy looking man; wearing a shirt with an otter on the front, sport shorts that don’t match his top and sandals of a hideous brown and orange colour, steps in to the room. “Hello Brothers Warner! I have a script I’ve written for a potential film, its a mix of Sci-Fi and horror” he exclaims confidently. One of the executives sighs “alright, lets see it”. “Can’t possibly be worse than what we’ve just put out” the other remarks. The executives flick through the script, reading the tale of an adventurer hacking her way through the lush forest of a strange, almost alien world. The adventurer encounters plenty of dangers; ducking a dragonfly the size of an eagle dive-bombing for her head, narrowly dodging the thrusting stinger of an ambushing scorpion the size of a large house-cat and throughout the film she is pursued by a large, unknown monster. At the films climax the adventurer, remarking that she had seen it all now, stumbles on what seems like a large log. She turns round and watches in horror as the log rises and squirms. The monster is revealed! It is a millipede, which rears up high enough to meet her eyes. It hisses, ready to lunge!

“Wow! This script has serious promise! even if it is a little cheesy” One of the executives gasps. “But where is it set?” The other enquires. “Oh turn to the last page!” The geek squeals excitedly. The executives do so, and see the words that would mark the twist ending to this film.

Earth. 300 Million Years Ago.

It sounds absurd, but believe it or not 300 Million years ago planet earth was home to these enormous arthropods. Meganuera and Pulmonoscorpius, the dragonfly and scorpion in our geek’s story, are both fascinating animals. However the most striking of these arthropods, and the one that I have a personal story of, is our monstrous millipede, which has the scientific name Arthropleura, meaning “jointed ribs”.

First discovered in 1854 by Jordan & Meyer, there are two known species of Arthropleura. While the smaller species was only a quarter of a metre in length the larger species was the biggest terrestrial arthropod of all time. Measuring just over two and a half metres long and nearly half a metre wide it was a size that modern millipedes could only dream of. It had a body plan of 30 jointed segments and each segment was covered by a relatively thin armour plating. Its large size meant that it would have had few natural predators (unless one of the large amphibians of the time got in a lucky shot). Since its discovery the main controversy surrounding Arthropleura was whether it was related to millipedes or centipedes, and as a result whether it was a herbivore or a carnivore. This is due to no mouth-parts having been found as of yet. While remains of giant club mosses had been found in some fossil remains a relatively recent study by Kraus showed that this was actually the shed skin of an Arthropleura that had been deposited on top of these club moss fragments. However despite this the common consensus at the moment is that it is a millipede relative and as such a herbivore, feeding on dead plant matter like modern millipedes do today (although its jaws would have given you a bad bite!). Arthropleura first evolved around 315 million years ago and went extinct 299 million years ago. The main reasons for its extinction being a combination of the disappearance of the coal swamps that it resided in and reduction in oxygen levels.

A fossil of the armour plates of Arthropleura from the Senckenberg Museum of Frankfurt in Germany. Image Credit: Ghedoghedo, https://commons.m.wikimedia.org/wiki/File:Arthropleura_armata.jpg

Unfortunately such an animal really was a product of its time. Arthropleura and other arthropods could only grow to such a size because of the special environment conditions present during the Carboniferous period. The earth had a much higher concentration of oxygen compared to modern times (almost 35%, compared to about 21% today). This suited arthropods in particular as it meant that they could take in more oxygen and therefore have more energy via respiration to use in growth. Arthropods take in oxygen via tiny tubes on the side of their bodies, known as trachea, or even directly through their skin. This system is nowhere near as efficient as true lungs are in terrestrial vertebrates. As a result the lower oxygen levels of today means that arthropods simply can’t get enough energy from respiration to maintain such large sizes. If Arthropleura was alive today it would not be able to survive in such a low oxygen environment. However a large concentration of oxygen isn’t the sole reason for their size, as a lack of competition and predation could also have helped.

Fossilised tracks of Arthropleura such as these are found in places in Scotland (e.g. the Island of Arran) and the USA. Image Credit: Ashley Dace, https://www.geograph.org.uk/photo/1994274

To finish this blog i’m going to share my personal encounter with an Arthropleura! Or rather, its fossilised tracks. When I was around the age of 11, during a family holiday in Scotland, I went to see the Arthropleura tracks at Crail. My young palaeontology mad self loved seeing these tracks, and there is a photo that proves it. So as it turns out this animal is indeed Scottish! (well half-Scottish technically, as fossils and trackways have also been found in the USA, but I like to think its Scottish). Arthropleura tracks are also a famous attraction of the island of Arran in Scotland. Whilst I did go to Arran during the first year of my university degree sadly my group didn’t have time to go see them.

So Arthropleura is yet another example of the weird creatures that evolution has produced, and a true marvel of the distant past. In my humble opinion, it would make a fantastic movie monster!

References/Further Reading

A great blog on Arthropleura written by Hans-Dieter Sues, a Paleontologist based at the National Museum of Natural History in Washington DC. The blog was published on National Geographic

Sues, Hans-Dieter,”Largest Land-Dwelling “Bug” of All Time”, National Geographic, Jan. 15, 2011, blog.nationalgeographic.org/2011/01/15/largest-land-dwelling-bug-of-all-time/

A blog, published on National Geographic, which gives futher background on the Carboniferous Period

National Geographic, “Carboniferous Period”, National Geographic, nationalgeographic.com/science/prehistoric-world/carboniferous/

A paper from 1992, written by Paul Pearson (Pearson 1992) and published in the Scottish Journal of Geology describing Arthropleura tracks (named Diplichnites cuithensis) from Fife, Scotland

Pearson, P. N. (1992). “Walking traces of the giant myriapod Arthropleura from the Strathclyde Group (Lower Carboniferous) of Fife.” Scottish Journal of Geology 28(2): 127-133.

Another paper, written by Ronald Martino & Stephen Greb (Martino & Greb 2009) and published in Journal of Paleontology, describing a set of Arthropleura tracks, this time from Kentucky, USA

Martino, R., & Greb, S. (2009). Walking trails of the giant terrestrial arthropod Arthropleura from the Upper Carboniferous of Kentucky. Journal of Paleontology, 83(1), 140-146. doi:10.1017/S0022336000058200

Is it a shrimp?! Is it a jellyfish?! No its Anomalocaris!

Science is not always static. Like any living species that has, currently or will exist it is constantly changing over time, with scientific theories evolving to fit the best available evidence. This phenomenon is prevalent throughout the scientific world but one area where it can be very clearly seen is in Palaeontology, where there are many instances of reconstructions of extinct life being very different in the past than they are today. One example of this is Megalosaurus, one of the first dinosaurs to be properly described by science. The original Victorian interpretation can be seen in a full scale model at the Crystal Palace in London. It is an impressive sculpture of a big hulking four legged lizard, portrayed as the Victorian scientists interpreted it, however it is nothing like the more graceful reconstruction nowadays. Another example, whose outdated model can be seen in the Sedgwick Museum in Cambridge, is the Carboniferous arthropod Megarachne (“Great Spider”). Once thought to be the largest spider that ever lived new research in 2005 found it to instead be a small to medium sized species of freshwater sea scorpion (though in my opinion this doesn’t make Megarachne any less unsettling to look at!)

However one of the most interesting cases of an extinct animal whose palaeontological reconstruction has changed greatly over time with new discoveries has to be Anomalocaris, whose name means “unusual shrimp”. Anomalocaris lived approximately 505 million years ago during the Early-Mid Cambrian. The Cambrian, from an evolutionary perspective, was prehistories equivalent of the European Renaissance of the 15th century. New body plans and weird evolutionary experiments were emerging during this period of earths history. Some animals from this time became extinct not long after they appeared. Some, like the trilobites, survived for an amazingly long period of time (trilobites appeared 520 million years ago in the Early Cambrian and went extinct during the Late Permian 250 million years ago – a longevity of 250 million years!) While others would eventually evolve and diversify into the main animal groups alive today; including the vertebrates which humans are a part of.

Anomalocaris is one of these weird wonders. It was discovered in 1892 by Joseph Frederick Whiteeves in the Burgess Shale formation, a fossil lagerstätten (which is a site where a high concentration of fossil material is preserved due to special environmental conditions) in Canada. The original fossil, named Anomalocaris canadensis, looked like a shrimp but with no clear headparts, hence its name of “unusual shrimp”. For a long time Anomalocaris was only known from this basic description and while it was definitely strange, its lifestyle was a complete mystery. Nineteen years later in 1911 the palaeontologist Charles Walcott, who is famous for his extensive work on the Burgess Shale, discovered a fossil of what seemed to be a primitive jellyfish. He gave it the name Peytoia nathorsti. Later, in 1928, Laggania cambria was discovered; the name given to a long bodied fossil that was interpreted as a relative of sea cucumbers.

Now you may be wondering “I thought you were going to be talking about Anomalocaris? Why have you wondered off topic to these random animals?” Well this is where the story gets interesting! In the early 1980s, nearly 90 years after Anomalocaris was first named, a palaeontologist working at the University of Cambridge by the name of Harry Whittington, an expert in Cambrian arthropods of the Burgess Shale, saw something that was truly astounding while preparing a fossil from the Burgess Shale. As he chipped away at the rock he saw two Anomalocaris “shrimps” attached to the head of a larger body not too dissimilar to Laggania. Not only that but a Peytoia fossilwas found to be attached to this same head. It soon became clear that Anomalocaris, Peytoia and Laggania were not separate species, but all part of one huge Cambrian animal, which was given the name Anomalocaris as that had been the original fossil part that had been found.

Anomalocaris was the top predator of its day. At around a metre in length it was the largest single animal the earth had ever seen at that point. After identifying its prey using large compound eyes, which gave it excellent eyesight comparable to modern day insects, it then used its prongs, once thought to be shrimps, to grab and hold its prey. Anomalocaris then held the prey close to its mouth-parts, once thought to be Peytonia, so the mouth parts could rip and break through the hard exoskeletons of trilobites and the soft bodies of other Cambrian arthropods that made up its prey. Anomalocaris swam via undulatory movements of their regularly arranged horizontal side flaps in the same manner that modern day soft bodied marine invertebrates do today. Anomalocarids as a group were widely successful, ranging across the globe from Canada to China and living from the Early to Middle Cambrian period. While most Anomalocarids were predators, another species has been described relatively recently in 2014 and named Tamisiocaris borealis (“sieve shrimp”). It had a very different lifestyle to Anomalocaris, possessing bristles on its prongs which it’s thought to have used in filter feeding, behaving rather like the baleen whales of today. This makes Tamisiocaris the earliest example of a large filter feeding animal known to science.

So Anomalocarids, the weird shrimps of the Cambrian, really are a fascinating group of arthropods. Once thought to be multiple separate animals, Anomalocaris and other Anomalocarids have instead been shown to be one of the weirdest of all Cambrian animals, and a true example of the evolutionary variety that has evolved on this planet.

The strange shrimp itself
Image credit: UNE photos, https://www.flickr.com/photos/unephotos/6786859303

EDIT: In this blog I state that Anomalocaris could “rip and break through the hard exoskeletons of trilobites”. This is actually wrong! A study in 2010, led by James Whitey Hagadorn from the the Denver Museum of Nature and Science, used 3-D models of Anomalocaris‘ mouth-parts to show that (a) it couldn’t close its mouth the whole way and (b) its mouth was too delicate to crush hard exoskeletons. As a result Anomalocaris would have fed mostly on soft bodied animals, and maybe even trilobites that had just moulted (and therefore had softer shells).

References/Further Reading

A blog, written by Ed Yong and published in Discover Magazine, about Anomalocaris

Yong, Ed, “The sharp eyes of Anomalocaris, a top predator that lived half a billion years ago”, Discover, Dec. 7, 2011, discovermagazine.com/the-sciences/the-sharp-eyes-of-anomalocaris-a-top-predator-that-lived-half-a-billion-years-ago#.XW6b8S5KjIU

The official Burgess Shale website page about Anomalocaris

“Anomalocaris canadensis”, burgess-shale.rom.on.ca/en/fossil-gallery/view-species.php?id=1

Vinther et. al. 2014 paper, published in the journal Nature, on the filter feeding Anomalocarid Tamisiocaris

Vinther, J., Stein, M., Longrich, N. et al. A suspension-feeding anomalocarid from the Early Cambrian. Nature 507, 496–499 (2014). https://doi.org/10.1038/nature13010

A wired article talking about the Hagadorn 2010 study that showed that Anomalocaris couldn’t eat hard bodied prey (USED FOR THE EDIT)

Mosher, Dave, “Giant Vicious-Looking Ancient Shrimp Was a Disappointing Wimp”, Wired, Mar. 11, 2010, wired.com/2010/11/anomalocaris-trilobite-bite/

Original Hagadorn study: Hagadorn, J. (2010). Putting Anomalocaris on a soft-food diet. 2010 GSA Denver Annual Meeting.